This invention is directed to improved electrochemical fuel cells. In particular, the present invention is directed to an improved phosphoric acid electrolyte used in electrochemical fuel cells. The United States Government has rights in this invention pursuant to Contract No. DEN-3-290 between the U. S. Department of Energy and Westinghouse Electric Corporation.
The term "fuel cell" is used in the art to denote a device, system or apparatus wherein chemical energy of a fluid combustible fuel, for example, hydrogen, carbon monoxide, or a hydrocarbon, is electrochemically converted to electrical energy to a non-sacrificial or inert electrode. The true fuel cell is adapted for continuous operation and is supplied with both fuel and oxidant from sources outside the cell proper. Such cells include at least two non-sacrificial or inert electrodes, functioning as anodes and cathodes, respectively, which are separated by an electrolyte which provides ionic conductants therebetween, conduction means for electrical connection between such anode and cathode external to said electrolyte, means for admitting a fluid fuel into dual contact with the anode and electrolyte and means for emitting a fluid oxidant into dual contact with the cathode and electrolyte. Where necessary or desired, the electrolyte compartment is divided into an anolyte compartment and a catholyte compartment by an ion-permeable partition or ion-exchange membrane. Thus, in each cell, a fluid fuel is passed to the anode and there oxidized electrochemically, giving up electrons to the anode, while a fluid oxidant is passed to the cathode and there reduced upon receiving electrons upon such cathode. Since the voltage developed by individual cell is low, it is usually preferable to employ relatively small cells to electrically connect large numbers of such cells in series or in both series and parallel.
Phosphoric acid fuel cells are known in the industry. Concentrated phosphoric acid has been used as an electrolyte for these types of fuel cells. However, concentrated phosphoric acid has very limited wetting properties and requires tediously long periods to permeate the electrodes after the cell is filled. In addition, the wetting ability of the concentrated phosphoric acid does not improve appreciably with increasing temperatures up to 190.degree. C. so that heating the cell does not decrease electrode wetting time. Many approaches have been attempted in resolving the wetting problem associated with concentrated phosphoric acid as electrolyte. Initial approaches to increase the wetting properties of concentrated phosphoric acid have used selected surfactants. These surfactants should be insoluable in the acid and should not adversely affect cell performance. In addition, these surfactants should produce the desired effect at very low concentrations (for example, less than 1%).
Common anionic surface active agents such as sodium or ammonium lauryl sulfate do not dissolve in concentrated phosphoric acid and are thus excluded from further consideration. An ammonium surfactants, such as lauryl dimethyl ammonium chloride, have sufficient solubility concentrated phosphoric acid but introduce a foreign anion into the system which may have adverse long term effects. Non-ionic surfactants, such as ethoxylated fatty alcohol, are also candidates from both the solubility and purity standpoints. These are normally liquids that dissolve readily in viscous phosphoric acid. However, the non-ionic materials tend to decompose and produce electrolyte darkening near 100.degree. C. Clearly, the selection of a surfactant which will increase the wetting properties of the concentrated phosphoric acid without affecting the resulting properties of the fuel cell remains a problem in the art. Applicants have discovered a suitable and economical solution to the wetting properties of the phosphoric acid electrolyte.